Second-order self-organization in coordination-driven self-assembly: exploring the limits of self-selection

Abstract

Self-organization during the self-assembly of a series of functionalized bispyridyl organic donors with complementary di-Pt(II) acceptors into supramolecular rhomboids and rectangles is explored. The connectivity and location of functional groups on the organic donors ensures that they do not interfere sterically or electronically with their respective binding sites. Carefully controlled reaction conditions are employed so that the only means of self-organization during self-assembly is through "second-order" effects arising from the distal functional groups themselves. With the selection of functionalized systems studied, the extent of second-order self-organization varies from essentially zero to quite pronounced.

Figure 1

Chemical structures of DB24C8 functionalized 120° bispyridyl donor 1, ferrocenyl functionalized 120° donor 2, dendronized 120° donors 3a–c, hydrophobic 180° donors 4a–c, hydrophilic 180° donors 5a-c, as well as 60° and 0° di-Pt(II) acceptors 6 and 7.

Figure 2

(A) Full ¹H NMR spectrum of the DB24C8/ferrocene mixed rhomboid self-assembly. (B) Full ³¹P NMR spectrum of the mixed assembly wherein a single, sharp ³¹P signal (inset) indicates the formation of discrete supramolecular products.

Figure 3

Full ESI mass spectrum of the complex mixture of DB24C8 and/or ferrocene functionalized supramolecular rhomboids The ESI-MS results show the presence of both homotopic and heterotopic assemblies and is suggestive of the preferred formation of homotopic species.

Figure 4

³¹P NMR spectra of (A) the [G1]–[G2] mixed dendron rhomboid assemblies and (B) the [G2]–[G3] mixed dendron rhomboid assemblies. Both spectra indicate that only discrete supramolecular species and no oligomeric are obtained, however, differences between homotopic and heterotopic assemblies cannot be resolved.

Figure 5

Full ESI mass spectra of (A) the [G1]-[G2] and (B) the [G2]-[G3] mixed assemblies. The different intensities of mass spectral peaks are suggestive of a statistical mixture of homotopic and heterotopic assemblies.

Figure 6

Full spectra and close-up views of the ³¹P NMR spectra of the C₆/DEG (A), C₁₂/TEG (B), and C₁₈/HEG (C) mixtures showing the relative contributions of purely hydrophobic (dark blue), purely hydrophilic (light blue) and mixed amphiphilic (red) metallacycles rectangles.

Figure 7

ESI-MS results for the C₆/DEG (A), C₁₂/TEG (B), and C₁₈/HEG (C) mixtures. The series of spectra show that with an increase in hydrophobic/hydrophilic chain length comes a decline in the presence of mixed functionality rectangles.

Figure 8

Full spectrum and close-up views of the ³¹P NMR spectra of the purely hydrophobic C₆/C₁₈ mixture (A) and purely hydrophilic DEG/HEG mixture (B).

Figure 9

ESI-MS results for the hydrophobic C₆/C₁₈ mixture (A) and hydrophilic DEG/HEG mixture (B).

Scheme 1

Potential second-order self-organization effects are investigated through the self-assembly of (A) unrelated DB24C8 and ferrocene functionalized bispyridyl donors 1 and 2 with 60° di-Pt(II) acceptor 6, (B) related though different generation dendronized 120° bispyridyl donors 3a and 3b (or 3b and 3c) with 60° di-Pt(II) acceptor 6, and (C) like sized hydrophobic (4a–c) and hydrophilic (5a–c) 180° bispyridyl donors with a 0° di-Pt(II) molecular clip. In each case the donor:donor:acceptor ratio is 1:1:2.

Scheme 2

Schematic representation of the four general types of self-organization that are possible when two ditopic bispyridyl donors are combined with a di-Pt(II) acceptor in a 1:1:2 ratio: “social,” exclusive formation of heterotopic metallacycles; “narcissistic,” exclusive formation of only homotopic metallacycles; “statistical” formation of homotopic and heterotopic assemblies; and “amplified” formation of slightly more homotopic than heterotopic metallacycles or vice versa (the ratio of 2:1:2 is only one representative example of such self-organization).

Scheme 3

Schematic representation of the pre-assembly aggregation phenomena that likely explains the trend toward greater self-organization upon increasing chain length in mixtures of hydrophobic (dark blue) and hydrophilic (light blue) bispyridyl donors and a di-Pt(II) acceptor.